Display error message if the content is not loaded in iFrame.

Hello everybody. I have a project with an iFrame that is loading a video from vimeo. It is working perfectly, but what I want to do is display a message (in any way possible) when the video cannot be loaded (for example: if there is no internet connection at the moment).
I assume this requires knowledge in web development too, so I would like to hear some advice if somebody already have done similar thing.
Thank you!